File I/O improvements

- Tracer now iterates over blocks in disk order.
- Packer copies files per directory, in a separate thread.
- Packer only copies files if they don't exist yet.
- Packer also copies file permissions.

blender_asset_tracer/tracer/__init__.py

@@ -24,7 +24,11 @@ def deps(bfilepath: pathlib.Path) -> typing.Iterator[result.BlockUsage]:
     """
 
     bfile = blendfile.open_cached(bfilepath)
-    for block in asset_holding_blocks(file2blocks.iter_blocks(bfile)):
+
+    # Sort the asset-holding blocks so that we can iterate over them
+    # in disk order, which is slightly faster than random order.
+    ahb = asset_holding_blocks(file2blocks.iter_blocks(bfile))
+    for block in sorted(ahb):
         yield from blocks2assets.iter_assets(block)